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tic wastewater in a five-step sequencing batch reactor (SBR) was investigated at different hydraulic residence times (HRTs). The nutrient removal process consisted of anaerobic (An), anoxic (Ax), oxic (Ox), anoxic (Ax), oxic (Ox), and setting phases. The sludge age (SRT) was kept constant at 10 days, while the HRT of each phase was varied. Effects of hydraulic residence time of each phase on COD, nitrogen (NH4-N) and phosphate (PO4-P) removal were investigated. The hydraulic residence time of each phase was varied at four different levels and the most suitable residence time, resulting in maximum overall nutrient removal was determined. The highest observed COD,nitrogen (NH4-N) and phosphate (PO4-P) removal efficiencies were 96,87 and 90%, respectively, which was obtained with An/Ax/Ox/Ax/Ox phase residence times of 2/1/4.5/1.5//1.5h.
© 2004 Elsevier Inc. All right reserves.

Keywords: Biological nutrient; Hydraulic residence time; Sequencing batch reactor

1. Introduction

Sequencing batch reactors (SBR) have been originally used for COD and phosphate removal from wastewaters [1-6]. Due to recent regulations on nutrient discharges sequential batch reactor systems have been modified to achieve nitrogen removal in addition to COD and phosphate removal. SBR treatment systems consist of the batch steps of fill, react, settle, decant and idle in a cyclic operation. The steps in the react cycle are adjusted to provide anaerobic, anoxic and aerobic phases in certain number and sequence for biological nutrient removal.
A number of studies have been reported in the literature on nutrient removal [7-15]. Colunga and Martinez [5] studied the effects of different phases in a biofilm SBR on COD, phosphate and ammonia nitrogen removal. The highest removal efficiencies of COD and PO4-P were obtained with a 12h cycle and a phase ratio of 37/63% for anaerobic/aerobic phases.
Umble and Ketchum [7] studied a sequencing batch reactor biological treatment of a municipal wastewater. A 12h total cycle time resulted in BOD5, TSS and NH4-N removals of 98, 90 and 89%, respectively.
Important process variables affecting performance of an SBR used for nutrient removal were investigated by Chang and Hao [8] and 98%, respectively, at a solids retention time of 10 days, with a total cycle time of 6h.
An algorithm for optimization of the total cycle length and phase distribution in order to minimize the effluent nitrogen content was developed by Andreottola et al. [11]. Optimum results have been obtained with 3.3h of anoxic and 4.2h of anaerobic phases. Effluent nitrate, nitrite and ammonium contents were 2.9, 0.04 and 0.06 mgl-1, respectively.
Chang et al. [12] carried out experimental studies in a small-scale SBR system to define important parameters affecting the nutrient removal performance. Maximum nitrogen and phosphorous removals were obtained with 1-3-2h of anaerobic – aerobic - anoxic phases. Final N and P concentrations of less than 2 mgl-1 were obtained.
Combined phosphorus and nitrogen removal in a biofilm sequencing batch reactor with four reaction phases of anaerobic/aerobic/anoxic/aerobic was investigated by Zuniga and Martinez [13]. The system was operated successfully with COD, phosphate and ammonia nitrogen removals of 89±1%, 75±15% and 87±10%, respectively.
Kargi and Uygur [14] studied nutrient removal from a synthetic wastewater using an SBR as a function of the sludge age. Ten days of sludge age was found to be optimal resulting in nutrient removals of 94, 84 and 70% for COD, NH4-N and PO4-P, respectively.
Effects of nutrient media composition (COD/N/P ratio) on nutrient removal performance of an SBR have also been investigated by Kargi and Uygur [15] using a statistical experiment design. COD/N/P ratio of 100/3.3/0.7 was found to be optimal resulting in COD, NH4-N and PO4-P removals of 97, 99 and 94%, respectively.
In neither one of the aforementioned studies